In medical imaging so-called “hybrid modalities” are becoming increasingly important, for example PET-CT, SPECT-CT, PET-MRI and SPECT-MRI. The meanings of these abbreviations are as follows:
PET: Positron Emission Tomography
CT: Computer Tomography
SPECT: Single Photon Emission Computed Tomography
MRI: Magnetic Resonance tomography
The advantage of these combinations is the connection of a modality with a high local resolution for imaging anatomy (especially MRI or CT) to a modality with high sensitivity (especially SPECT or PET) for imaging/detecting biological processes, including cellular and metabolic processes.
In the case of MRI, locating an array of radiation imaging detectors inside the bore of a MRI system presents to the designer many technical challenges, which are exacerbated not only by spatial constraints, magnetic and gradient fields, but also by ohmic and eddy current heating. In general, the magnetic field strength and spatial constraints of a MRI system generally limit optical sensor selection to solid-state types, such as avalanche photo diodes (APDs) and silicon photo multipliers (SiPMs). However, most conventional PET and PETCT detectors rely on photo multiplier tubes (PMTs) as the primary optical sensing element since they are generally insensitive to temperature fluctuations since their photon conversion elements are incased within an evacuated tube. Unfortunately, most PMTs are highly susceptible to changes in magnetic fields. Further, most conventional PMTs for PET and PETCT detectors are relatively large (>100 mm). Accordingly, PMTs are generally impractical for use in the spatially restricted, magnetic environment of an MRI system.
In contrast, solid state photon sensors, such as APDs and SiPMs, are relatively small in height (1-2 mm) and are typically unaffected by the magnetic fields of an MRI system. However, the gain of such sensors can significantly fluctuate in response to temperature fluctuations of the ambient environment. Therefore, when APDs and SiPMs are used to form an optical sensing element for a PET detector, such gain fluctuations can be very detrimental to the operation of the PET detector, as it generally requires long term stability in the optical sensing element. Therefore, a PET detector designed for use in an MRI requires proper cooling if incorporating either APDs or SiPMs as its optical conversion element.